The name "Starplus" was assigned to MANY MANY different systems produced by Vodavi - a more exact description may yield a more exact response. That said, with the more current versions, admin access was only granted to X100 unless allowed within programming. So, first task is to be SURE that you're on X100. I always confirm at frame. Not sure how it would work if extension numbers have been swapped within software. If no joy, try on first digital station port at frame.
If still no joy, try accessing maint via PC/terminal - I don't think that this password can be changed.
If still no joy, default system via DIP 8